The present invention relates to static inverters for providing a high frequency alternating current power supply, and more particularly to inverters for supplying loads which may have widely and rapidly varying impedance.
Static inverters use switching thyristors, which may be arranged in various configurations, and which are switched ON and OFF in a predetermined sequence to provide an alternating current output of frequency determined by the switching rate. Such inverters are used to provide a high frequency power supply from a direct current source, or through rectifiers from a conventional power line. Power supplies of this type can be used for many purposes but they are particularly useful for supplying induction heating equipment in frequency ranges of the order of 10 kilohertz, for example, because of the relative ease with which the frequency can be varied.
Induction heating coils such as are used for heat treating, melting and similar purposes are, of course, highly inductive and for this reason, it is necessary to provide a capacitor in parallel with such coils to raise the power factor to an acceptable value. A tank circuit is thus formed which is preferably operated at or near parallel resonance. In the use of such equipment, however, the magnetic characteristics of the work, or other conditions, may vary during operation so that the inductance and resistance of the induction heating coil change, sometimes quite rapidly, which has the effect of changing the impedance of the tank circuit and the resonant frequency. This, of course, changes the current and power dissipation in the coil and makes it necessary to control the power. This has often been done by varying the voltage or the frequency supplied to the tank circuit.
Static inverters are highly suitable as power supplies for induction heating equipment, because the frequency can readily be varied by controlling the firing times of the switching thyristors as required. Conventional inverters, however, involve certain problems such as that of turning OFF the thyristors at the desired times. This is usually done by means of commutating capacitors which apply a reverse voltage to turn the thyristor OFF. Thyristors require a definite minimum turn-off time, however, to fully regain their blocking characteristics, and the reverse voltage must be maintained at least for this length of time. This requirement places a definite limitation on the maximum frequency that can be obtained, and the use of commutating capacitors may also involve the possibility of undesirably high reverse voltages. Certain known types of static inverters are shown, for example, in U.S. patents to Landis U.S. Pat. No. 3,566,243, Duff et al. U.S. Pat. No. 3,621,366 and Brown U.S. Pat. No. 3,859,587.